Carbon nanostructures manufactured from catalytic templating nanoparticles

ABSTRACT

Methods for manufacturing carbon nanostructures include: 1) forming a plurality of catalytic templating particles using a plurality of dispersing agent molecules; 2) forming an intermediate carbon nanostructure by polymerizing a carbon precursor in the presence of the plurality of templating nanoparticles; 3) carbonizing the intermediate carbon nanostructure to form a composite nanostructure; and 4) removing the templating nanoparticles from the composite nanostructure to yield the carbon nanostructures. The carbon nanostructures are well-suited for use as a catalyst support. The carbon nanostructures exhibit high surface area, high porosity, and high graphitization. Carbon nanostructures according to the invention can be used as a substitute for more expensive and likely more fragile carbon nanotubes.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of co-pending U.S. patent applicationSer. No. 11/539,042, filed Oct. 5, 2006, which claims the benefit under35 U.S.C. §119 of U.S. provisional application Ser. No. 60/724,323,filed Oct. 6, 2005, and also of U.S. provisional application Ser. No.60/724,315, filed Oct. 6, 2005. The disclosures of the foregoingapplications are incorporated herein in their entirety.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates generally to carbon nanomaterials. Moreparticularly, the present invention relates to carbon nanostructuresthat are manufactured using a carbon precursor and a catalytictemplating particle.

2. The Related Technology

Carbon materials have been used in a variety of fields ashigh-performance and functional materials. Pyrolysis of organiccompounds is well-known to be one of the most useful methods to preparecarbon materials. For example, carbon materials can be produced bypyrolyzing resorcinol-formaldehyde gel at temperatures above 600° C.

Most carbon materials obtained by pyrolysis of organic compounds attemperatures between 600-1400° C. tend to be amorphous or have adisordered structure. Obtaining highly crystalline or graphitic carbonmaterials can be very advantageous because of the unique propertiesexhibited by graphite. For example, graphitic materials can beconductive and form unique nanomaterials such as carbon nanotubes.However, using existing methods it is difficult to make thesewell-crystallized graphite structures using pyrolysis, especially attemperatures less than 2000° C.

To acquire the graphite structure at lower temperature many studies havebeen carried out on carbonization in the presence of a metal catalyst.The catalyst is typically a salt of iron, nickel, or cobalt that ismixed with carbon precursor. Using catalytic graphitization, graphiticmaterials can be manufactured at temperatures between 600° C. and 1400°C. Most catalytic graphitization methods have focused on making graphitenanotubes. However, the yield of crystalline materials is still very low(e.g., for carbon nanotubes the yield is less than 2%). These low yieldsmake it difficult to use the nanomaterials in making useful articles.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to novel methods for manufacturing carbonnanostructures using a carbon precursor and a catalyst. The carbonnanostructures are formed around a plurality of templatingnanoparticles. In an exemplary embodiment, the templating nanoparticlesare manufactured from catalytic metal atoms using an organic dispersingagent. The catalytic nanoparticles advantageously function both as anucleating site for carbon nanostructure formation and as a catalystduring carbonization and/or polymerization of the carbon precursor.

The novel methods of making carbon nanostructures according to thepresent invention can include all or a portion of the following steps:

-   (i) forming a plurality of catalytic templating nanoparticles by:    -   (a) reacting a plurality of precursor catalyst atoms with a        plurality of organic dispersing agent molecules to form        complexed catalyst atoms; and    -   (b) allowing or causing the complexed catalyst atoms to form the        templating nanoparticles;-   (ii) forming one or more intermediate carbon nanostructures by    polymerizing a carbon precursor in the presence of the templating    nanoparticles;-   (iii) carbonizing the intermediate carbon nanostructures to form a    plurality of composite nanostructures; and-   (iv) removing the templating nanoparticles from the composite    nanostructures to yield the carbon nanostructures.

In the method of the present invention, the dispersed templatingnanoparticles are formed using a dispersing agent. The dispersing agentis an organic molecule that includes one or more functional groups thatcan bond with the catalyst atoms. In a preferred embodiment, the one ormore functional groups comprise a hydroxyl, a carboxyl, a carbonyl, anamine, an amide, a nitrile, a nitrogen with a free lone pair ofelectrons, an amino acid, a thiol, a sulfonic acid, a sulfonyl halide,an acyl halide, or a combination of any of these. The dispersing agentmolecules bond with the catalyst atoms to form a complex. The complexedcatalyst atoms then react or agglomerate to form solid catalytictemplating particles. The organic dispersing agent can control theformation, size, and/or dispersion of the catalytic templatingnanoparticles.

In the method of the present invention, the catalytic templatingnanoparticles are used as a template for making the nanostructures. Whenmixed with the carbon precursor, the templating nanoparticles provide anucleation site where carbonization and/or polymerization can begin orbe enhanced. Because the templating nanoparticles are made fromcatalytic atoms, the templating particles can advantageously serve asboth a nucleating site and as a catalyst for carbonization and/orpolymerization. This feature of the invention eliminates the need to addtemplating particles and catalyst separately (e.g., silica sol and metalsalts). In this manner solid catalytic templating particles avoid thesituation where the separately added catalyst atoms undesirably act as anucleation site. The catalytic templating nanoparticles of the presentinvention can advantageously produce carbon nanostructures having moreuniform features (e.g., inner hole diameter) than carbon nanostructuresmanufactured using existing methods.

In an exemplary embodiment, the method of the present invention producescarbon nanostructures having a ring shape. The ring shape can give thecarbon nanostructures beneficial properties such as high porosity andhigh surface area. Beneficial features such as these make the carbonnanostructures useful as a support material for a fuel cell catalyst.The high surface area allows for high metal loadings while the highporosity improves the performance of the fuel cell catalyst due toimproved diffusion of reactants. Their high electrical conductivityallows the nanostructures to be used in the anode or the cathode of afuel cell. Carbon nanostructures can be substituted for carbonnanotubes, which are typically more expensive and likely more fragile.

These and other advantages and features of the present invention willbecome more fully apparent from the following description and appendedclaims as set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the above and other advantages and features of thepresent invention, a more particular description of the invention willbe rendered by reference to specific embodiments thereof which areillustrated in the appended drawings. It is appreciated that thesedrawings depict only typical embodiments of the invention and aretherefore not to be considered limiting of its scope. The invention willbe described and explained with additional specificity and detailthrough the use of the accompanying drawings in which:

FIG. 1A is a high resolution TEM image of a plurality of carbonnanostructures formed according to an exemplary embodiment of thepresent invention;

FIG. 1B is a high resolution TEM image showing a close-up of variouscarbon nanostructures of FIG. 1A;

FIG. 1C is a high resolution TEM image showing yet a closer image of acarbon nanostructure of FIG. 1A;

FIG. 2A is a high resolution TEM image of a plurality of carbonnanostructures formed according to an exemplary embodiment of thepresent invention;

FIG. 2B is a high resolution TEM image showing a closer image of variouscarbon nanostructures of FIG. 2A;

FIG. 3A is a high resolution TEM image of a plurality of carbonnanostructures formed according to an exemplary embodiment of thepresent invention;

FIG. 3B is a high resolution TEM image showing a closer image of variouscarbon nanostructures of FIG. 3A;

FIG. 4A is a high resolution SEM image of carbon nanostructures formedaccording to an exemplary embodiment of the present invention showingthem to be sphere-like in shape; and

FIG. 4B is a high resolution SEM image showing a closer image of variouscarbon nanostructures of FIG. 4A.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS I. Introduction andDefinitions

The present invention is directed to methods of making carbonnanostructures and the use of the carbon nanostructures as catalystsupports (e.g., for fuel cell catalysts). Methods for manufacturingcarbon nanostructures generally include 1) forming a plurality of solidcatalytic templating particles by reacting catalyst atoms with anorganic dispersing agent, 2) forming intermediate carbon nanostructuresby polymerizing a carbon precursor in the presence of the templatingnanoparticles, 3) carbonizing the intermediate carbon nanostructures toform composite nanostructures, and 4) removing the templatingnanoparticles from the composite nanostructures to leave carbonnanostructures. The carbon nanostructures manufactured using theforegoing steps have one or more carbon layers forming a wall thatgenerally appears to define a carbon nanoring or truncated tube-likestructure when viewed as TEM images but which might be characterized ashollow but irregular multi-walled sphere-like (or spheroidal)nanostructures when the TEM images are analyzed in combination with SEMimages of the same material. In one embodiment, the carbonnanostructures form clusters of grape-like structures as seen in SEMimages but which are known to be hollow multi-walled nanostructures asshown by TEM images of the same material.

For purposes of the present invention, a precursor catalyst material isany material that can appreciably increase the rate of carbonization ofthe carbon precursor when combined therewith. Non-limiting examples ofprecursor catalyst materials include iron, cobalt, and/or nickel.

Solid catalyst templating particles are particles where substantiallyall of the templating particle are made from one or more catalyticmaterials.

II. Components Used to Manufacture Carbon Nanostructures

The following exemplary components can be used to carry out the abovementioned steps for manufacturing carbon nanostructures according to thepresent invention.

A. Polymerizable Carbon Precursor

Any type of carbon material can be used as the carbon precursor of thepresent invention so long as it can disperse the templating particles,polymerize to form an intermediate nanostructure, and become carbonizedby heat-treatment. Suitable compounds include single and multi-ringaromatic compounds such as benzene and naphthalene derivatives that havepolymerizable functional groups. Also included are ring compounds thatcan form single and multi-ring aromatic compounds upon heating.Functional groups that can participate in polymerization include COOH,C═O, OH, C═C, SO₃, NH₂, SOH, N═C═O, and the like.

The polymerizable carbon precursor can be a single type of molecule(e.g., a compound that can polymerize with itself), or the polymerizablecarbon precursor can be a combination of two or more different compoundsthat co-polymerize together. For example, in an exemplary embodiment,the carbon precursor can be a resorcinol-formaldehyde gel. In this twocompound embodiment, the formaldehyde acts as a cross-linking agentbetween resorcinol molecules by polymerizing with the hydroxyl groups ofthe resorcinol molecules.

Other examples of suitable polymerizable precursor materials includeresorcinol, phenol resin, melamine-formaldehyde gel, poly(furfurylalcohol), poly(acrylonitrile), sucrose, petroleum pitch, and the like.Other polymerizable benzenes, quinones, and similar compounds can alsobe used as carbon precursors and are known to those skilled in the art.

In an exemplary embodiment, the carbon precursor is a hydrothermallypolymerizable organic compound. Suitable organic compounds of this typeinclude citric acid, acrylic acid, benzoic acid, acrylic ester,butadiene, styrene, cinnamic acid, and the like.

B. Catalytic Templating Nanoparticles

As described below, the formation of the catalytic templating particlesgenerally includes reacting a plurality of templating catalyst atomswith a plurality of dispersing agent molecules in a solvent to formcomplexed catalyst atoms. The complexed catalyst atoms then react toform nanoparticles.

1. Carbon Precursor Catalyst Atoms

The precursor catalyst atom can be any material that can cause orpromote carbonization and/or polymerization of the carbon precursor. Ina preferred embodiment, the catalyst is a transition metal catalystincluding but not limited to iron, cobalt, or nickel. These transitionmetal catalysts are particularly useful for catalyzing many of thepolymerization and/or carbonization reactions involving the carbonprecursors described above.

2. Dispersing Agents

In addition to catalyst atoms, the catalyst complexes of the presentinvention include one or more dispersing agents. The dispersing agent isselected to promote the formation of nanocatalyst particles that have adesired stability, size and/or uniformity. Dispersing agents within thescope of the invention include a variety of small organic molecules,polymers and oligomers. The dispersing agent is able to interact andbond with catalyst atoms dissolved or dispersed within an appropriatesolvent or carrier through various mechanisms, including ionic bonding,covalent bonding, Van der Waals interaction/bonding, lone pair electronbonding, or hydrogen bonding.

To provide the bonding between the dispersing agent and the catalystatoms, the dispersing agent includes one or more appropriate functionalgroups. Preferred dispersing agents include functional groups which haveeither a charge or one or more lone pairs of electrons that can be usedto complex a metal catalyst atom, or which can form other types ofbonding such as hydrogen bonding. These functional groups allow thedispersing agent to have a strong binding interaction with the catalystatoms.

The dispersing agent may be a natural or synthetic compound. In the casewhere the catalyst atoms are metal and the dispersing agent is anorganic compound, the catalyst complex so formed may be anorganometallic complex.

In an exemplary embodiment, the functional groups of the dispersingagent comprise one or more members selected from the group of ahydroxyl, a carboxyl, a carbonyl, an amine, an amide, a nitrile, anitrogen with a free lone pair of electrons, an amino acid, a thiol, asulfonic acid, a sulfonyl halide, or an acyl halide. The dispersingagent can be monofunctional, bifunctional, or polyfunctional.

Examples of suitable monofunctional dispersing agents include alcoholssuch as ethanol and propanol and carboxylic acids such as formic acidand acetic acid. Useful bifunctional dispersing agents include diacidssuch as oxalic acid, malic acid, malonic acid, maleic acid, succinicacid, and the like; dialcohols such as ethylene glycol, propyleneglycol, 1,3-propanediol, and the like; hydroxy acids such as glycolicacid, lactic acid, and the like. Useful polyfunctional dispersing agentsinclude sugars such as glucose, polyfunctional carboxylic acids such ascitric acid, pectins, cellulose, and the like. Other useful dispersingagents include ethanolamine, mercaptoethanol, 2-mercaptoacetate, aminoacids, such as glycine, and sulfonic acids, such as sulfobenzyl alcohol,sulfobenzoic acid, sulfobenzyl thiol, and sulfobenzyl amine. Thedispersing agent may even include an inorganic component (e.g.,silicon-based).

Suitable polymers and oligomers within the scope of the inventioninclude, but are not limited to, polyacrylates, polyvinylbenzoates,polyvinyl sulfate, polyvinyl sulfonates including sulfonated styrene,polybisphenol carbonates, polybenzimidizoles, polypyridine, sulfonatedpolyethylene terephthalate. Other suitable polymers include polyvinylalcohol, polyethylene glycol, polypropylene glycol, and the like.

In addition to the characteristics of the dispersing agent, it can alsobe advantageous to control the molar ratio of dispersing agent to thecatalyst atoms in a catalyst suspension. A more useful measurement isthe molar ratio between dispersing agent functional groups and catalystatoms. For example, in the case of a divalent metal ion two molarequivalents of a monovalent functional group would be necessary toprovide the theoretical stoichiometric ratio. In a preferred embodiment,the molar ratio of dispersing agent functional groups to catalyst atomsis preferably in a range of about 0.01:1 to about 100:1, more preferablyin a range of about 0.05:1 to about 50:1, and most preferably in a rangeof about 0.1:1 to 20:1.

The dispersing agents of the present invention allow for the formationof very small and uniform nanoparticles. In general, the nanocatalystparticles formed in the presence of the dispersing agent are less than 1micron in size. Preferably the nanoparticles are less than 100 nm, morepreferably less than 50 nm and most preferably less than 20 nm.

During pyrolysis of the carbon precursor, the dispersing agent caninhibit agglomeration and deactivation of the catalyst particles. Thisability to inhibit deactivation can increase the temperature at whichthe nanocatalysts can perform and/or increase the useful life of thenanocatalyst in the extreme conditions of pyrolysis. Even if includingthe dispersing agent only preserves catalytic activity for a fewadditional milliseconds, or even microseconds, the increased duration ofthe nanocatalyst can be very beneficial at high temperatures, given thedynamics of carbonization.

3. Solvents and Other Additives

The liquid medium in which the catalytic templating nanoparticles areprepared may contain various solvents, including water and organicsolvents. Solvents participate in particle formation by providing aliquid medium for the interaction of catalyst atoms and dispersingagent. In some cases, the solvent may act as a secondary dispersingagent in combination with a primary dispersing agent that is not actingas a solvent. In one embodiment, the solvent also allows thenanoparticles to form a suspension. Suitable solvents include water,methanol, ethanol, n-propanol, isopropyl alcohol, acetonitrile, acetone,tetrahydrofuran, ethylene glycol, dimethylformamide, dimethylsulfoxide,methylene chloride, and the like, including mixtures thereof.

The catalyst composition can also include additives to assist in theformation of the nanocatalyst particles. For example, mineral acids andbasic compounds can be added, preferably in small quantities (e.g., lessthan 5 wt %). Examples of mineral acids that can be used includehydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, and thelike. Examples of basic compounds include sodium hydroxide, potassiumhydroxide, calcium hydroxide, ammonium hydroxide, and similar compounds.

It is also possible to add solid materials to assist in nanoparticleformation. For example, ion exchange resins may be added to the solutionduring catalyst formation. Ion exchange resins can be substituted forthe acids or bases mentioned above. Solid materials can be easyseparated from the final iron catalyst solution or suspension usingsimple techniques such as centrifugation and filtration.

III. Manufacturing Carbon Nanostructures

The carbon nanostructures of the present invention can be manufacturedusing all or a portion of the following steps: (i) forming a pluralityof dispersed catalytic templating nanoparticles by reacting a pluralityof precursor catalyst atoms with a plurality of dispersing agentmolecules, (ii) mixing the plurality of catalytic templatingnanoparticles (e.g., iron particles) with a carbon precursor (e.g.,citric acid) and allowing or causing the carbon precursor to polymerizeto form a plurality of intermediate nanostructures, (iii) carbonizingthe intermediate nanostructures to form a plurality of compositenanostructures, and (iv) removing the templating nanoparticles from theplurality of composite nanostructures to yield the carbonnanostructures.

A. Providing Catalytic Templating Nanoparticles

The process for manufacturing the nanoparticles can be broadlysummarized as follows. First, one or more types of precursor catalystatoms and one or more types of dispersing agents are selected. Second,the precursor catalyst atoms (e.g., in the form of a ground state metalor metal salt) and dispersing agent (e.g., in the form of a carboxylicacid or its salt) are reacted or combined together to form catalystcomplexes. The catalyst complexes are generally formed by firstdissolving the catalyst atoms and dispersing agent in an appropriatesolvent and then allowing the catalyst atoms to bond with the dispersingagent molecules. The various components may be combined or mixed in anysequence or combination. In addition, a subset of the components can bepremixed prior to addition of other components, or all components may besimultaneously combined.

In one aspect of the invention, the catalyst complexes may be consideredto be the complexed catalyst atoms and dispersing agent, exclusive ofthe surrounding solvent. Indeed, it is within the scope of the inventionto create catalyst complexes in a solution and then remove the solventto yield dried catalyst complexes. The dried catalyst complexes can bereconstituted by adding an appropriate solvent.

In an exemplary embodiment, the components are mixed for a period ofabout 1 hour to about 14 days. This mixing is typically conducted attemperatures ranging from 0° C. to 200° C. Preferably the temperaturedoes not exceed 100° C.

The precursor catalyst atoms are typically provided in the form of aniron salt such as iron chloride, iron nitrate, iron sulfate, or thelike. These compounds are often soluble in an aqueous solvent. Formationof the catalyst nanoparticles using metal salts can lead to theformation of additional by-products from the release of the anion. Ifdesired, formation of an anion can be avoided by using a metal powder(e.g., iron). Typically the only significant by-product of the catalystpreparation using iron metal is hydrogen gas, which is evolved duringthe mixing procedure. If the catalyst particles are made using amaterial that evolves hydrogen gas or another gas, the mixture istypically vented and/or exposed to air periodically (or continuously)during the preparation procedure.

In an exemplary embodiment, the nanocatalyst particles are in an activeform once the mixing step is complete or upon further reduction usinghydrogen, for example. In a preferred embodiment, the nanocatalystparticles are formed as a suspension of stable active metal nanocatalystparticles. The stability of the nanocatalyst particles prevents theparticles from agglomerating together and maintains them in suspension.Even where some or all of the nanocatalyst particles settle out ofsolution over time, the nanocatalyst particles can be easilyre-suspended by mixing.

A base can be added (e.g., concentrated aqueous ammonia) to adjust thepH of the solution to between about 8 and about 13, and more preferablybetween about 10 and about 11. The higher pH can be useful forprecipitating the precursor catalyst atoms in a finely divided manner.

The catalytic templating nanoparticles are capable of catalyzingpolymerization and/or carbonization of the carbon precursor. Theforegoing procedure for preparing the catalytic templating particles canassist in arranging the catalyst atoms into particles that arecatalytically active. In contrast, the inventors have found that somecommercially available reagents (e.g., at least one commerciallyavailable iron citrate) do not have satisfactory catalytic activity.

B. Polymerizing the Carbon Precursor

The catalytic templating nanoparticles are mixed with a carbon precursor(e.g., citric acid) under conditions suitable for the carbon precursorto polymerize around the templating nanoparticles. Because thetemplating nanoparticles are catalytically active, the templatingnanoparticles can preferentially accelerate and/or initiatepolymerization of the carbon precursor near the surface of thetemplating particles.

The concentration of catalytic templating nanoparticles in the carbonprecursor is typically selected to maximize the number of carbonnanostructures formed while still producing uniformly shapednanostructures. The amount of catalytic templating particles can varydepending on the type of carbon precursor being used. In an exemplaryembodiment the molar ratio of carbon precursor to catalyst atoms isabout 0.1:1 to about 100:1, more preferably about 1:1 to about 30:1.

The precursor composition is allowed to cure for sufficient time suchthat a plurality of intermediate carbon nanostructures are formed aroundthe templating nanoparticles. The time needed to form intermediatenanostructures depends on the temperature, the type and concentration ofthe catalyst material, the pH of the solution, and the type of carbonprecursor being used. During polymerization, the intermediate carbonnanostructures can be individual organic structures or an association ofnanostructures that break apart during carbonization and/or removal ofamorphous carbon.

Ammonia added to adjust the pH can also effect polymerization byincreasing the rate of polymerization and by increasing the amount ofcross linking that occurs between precursor molecules.

For hydrothermally polymerizable carbon precursors, polymerizationtypically occurs at elevated temperatures. In a preferred embodiment,the carbon precursor is heated to a temperature of about 0° C. to about200° C., and more preferably between about 25° C. to about 120° C.

An example of a suitable condition for polymerization ofresorcinol-formaldehyde gel (e.g., with iron particles and a solution pHof 1-14) is a solution temperature between 0° C. and 90° C. and a curetime of less than 1 hour to about 72 hours. Those skilled in the art canreadily determine the conditions necessary to cure other carbonprecursors under the same or different parameters.

In an exemplary embodiment the polymerization is not allowed to continueto completion. Terminating the curing process before the entire solutionis polymerized can help to form a plurality of intermediatenanostructures that will result in individual nanostructures, ratherthan a single mass of carbonized material. However, the presentinvention includes embodiments where the carbon precursor forms aplurality of intermediate carbon nanostructures that are linked orpartially linked to one another. In this embodiment, individualnanostructures are formed during carbonization and/or during the removalof amorphous carbon.

Forming intermediate carbon nanostructures from the dispersion oftemplating nanoparticles causes formation of a plurality of intermediatecarbon nanostructures having unique shapes and sizes. Ultimately, theproperties of the nanostructure depend at least in part on the shape andsize of the intermediate carbon nanostructure. Because of the uniqueshapes and sizes of the intermediate carbon nanostructures, the finalnanostructures can have beneficial properties such as high surface areaand high porosity, among others.

C. Carbonizing the Intermediate Nanostructures

Once the intermediate nanostructures are obtained, they are carbonizedby heating to produce carbonized composite nanostructures. In anexemplary embodiment, the intermediate nanostructures are heated to atemperature between about 500° C. and about 2500° C. During the heatingprocess, atoms such as oxygen and nitrogen are volatilized or otherwiseremoved from the intermediate nanostructure and the carbon atoms arerearranged or coalesced to form a carbon-based structure.

In a preferred embodiment, the carbonizing step produces a graphitebased nanostructure. The graphite based nanostructure has carbon atomsarranged in sheets of sp² hybridized carbon atoms. The graphitic layerscan provide unique and beneficial properties, such as electricalconduction and structural strength and/or rigidity.

D. Removing the Templating Nanoparticles and/or Amorphous Carbon toYield Carbon Nanostructures

In a final step, the templating nanoparticles and/or extraneousamorphous (i.e., non-graphitic) carbon are removed from the compositenanostructures. Typically, the templating nanoparticles are removedusing acids or bases such as nitric acid, hydrogen fluoride, or sodiumhydroxide. The method of removing the templating nanoparticles oramorphous carbon depends on the type of templating nanoparticle orcatalyst atoms in the composite. Catalyst atoms or particles (e.g., ironparticles or atoms) can typically be removed by refluxing the compositenanostructures in 5.0 M nitric acid solution for about 3-6 hours.

Any removal process can be used to remove the templating nanoparticlesand/or amorphous carbon so long as the removal process does notcompletely destroy the carbon nanospheroidal and/or nanoring structure.In some cases it can be beneficial to at least partially remove some ofthe carbonaceous material from the intermediate nanostructure during theremoval process. It is not presently known at what point in the methodthat the annular shape is formed, whether it is during thepolymerization step, carbonation step, or nanoparticle removal step.

IV. Carbon Nanostructures

The methods of the present invention produce a multi-walled carbonnanostructure having useful properties such as unique shape, size, andelectrical properties. In a preferred embodiment, the carbonnanostructures can be a regular or irregularly shaped annular structurehaving a hole therethrough (i.e., a nanoring or hollow multi-walled,sphere-like or spheroidal structure). The carbon nanostructures of thepresent invention are particularly advantageous for some applicationswhere high porosity, high surface area, and/or a high degree ofgraphitization are desired. Carbon nanostructures manufactured as setforth herein can be substituted for carbon nanotubes, which aretypically far more expensive.

The size of the nanostructure is determined in large part by the size ofthe templating nanoparticles used to manufacture the carbonnanostructures. Because the carbon nanostructures form around thetemplating nanoparticles, the hole or inner diameter of the carbonnanostructures typically corresponds to the outer diameter of thetemplating nanoparticles. The inner diameter of the carbonnanostructures can be between 0.5 nm to about 90 nm. For certainapplications such as fuel cells, the inner diameter is preferablybetween about 1 nm and about 50 nm.

FIGS. 1A-1C, 2A-2B, and 3A-3B show TEM images of exemplary carbonnanostructures made according to the methods of the present invention,the details of which are described in Example 1 below. FIGS. 4A-4B showSEM images of exemplary nanostructures made according to the presentinvention, the details of which are described in Example 1 below.

The generally annular shape of the carbon nanostructures is shown in theTEM images of FIGS. 1A-1C, 2A-2B, and 3A-3B. The generally sphere-likeshape of the carbon nanostructures is shown in the SEM images of FIGS.4A-4B. In many of the carbon nanostructures shown in the TEM images, theouter ring diameter is between about 10 nm and about 60 nm and the poresize is about 10 nm to about 40 nm. However, the present inventionincludes nanostructures having larger and smaller diameters. Typically,the carbon nanostructures have an outer diameter that is less than about100 nm to maintain structural integrity.

The thickness of the nanostructure wall is measured from the insidediameter of the wall to the outside diameter of the wall. The thicknessof the nanostructure can be varied during manufacture by limiting theextent of polymerization and/or carbonization of the carbon precursor asdescribed above. Typically, the thickness of the carbon nanostructurewall is between about 1 nm and 20 nm. However, thicker and thinner wallscan be made if desired. The advantage of making a thicker wall isgreater structural integrity. The advantage of making a thinner wall isgreater surface area and porosity.

The wall of the carbon nanostructure can also be formed from multiplegraphitic layers. The TEM images in FIGS. 1A, 1B, and 1C clearly showsmultiple layers. In an exemplary embodiment, the carbon nanostructureshave walls of between about 2 and about 100 graphite layers, morepreferably between about 5 and 50 graphite layers and more preferablybetween about 5 and 20 graphite layers. The number of graphitic layerscan be varied by varying the thickness of the carbon nanostructure wallas discussed above. The graphitic characteristic of the carbonnanostructures is believed to give the carbon nanostructures beneficialproperties that are similar to the benefits of multi-walled carbonnanotubes (e.g., excellent conductivity). They can be substituted forcarbon nanotubes and used in virtually any application where carbonnanotubes can be used but often with predictably superior results.

The carbon nanostructures also have a desired length. The length of thecarbon nanostructure is the length of the hole as measure along the axisof the hole. If the carbon nanostructure is lying flat or horizontal,the length of the carbon nanostructure is the height of the carbonnanostructure. In a preferred embodiment, the length of the carbonnanostructure is limited by forming the carbon nanostructures fromsubstantially spherical templating nanoparticles. Carbon nanostructuresformed from spherical templating nanoparticles typically only have alength that is approximately the same as the outer diameter of thecarbon nanostructure. Such a result can be obtained because of thesubstantially even polymerization and/or carbonization about thetemplating nanoparticle. With regard to what appear to be carbonnanorings in the TEM images, the length typically does not exceed theouter diameter of the carbon nanoring because the length and the outerdiameter typically grow at substantially the same rate duringpolymerization. Carbon nanostructures that have a length that is lessthan or about equal to the outer diameter can be advantageous because oftheir large surface area and/or because they can better facilitatediffusion of reactants and reaction products as compared to, e.g.,carbon nanotubes.

Another feature of the carbon nanostructures of the present invention isthe formation of a non-tubular wall. As shown in the TEM images of FIGS.1A, 1B, and 1C, and also the SEM images of FIGS. 4A and 4B, thegraphitic layers form a substantially solid wall. This is in contrast toattempts by others to make a carbon nanostructure where the ends of acarbon nanotube are connected to make a ring. Carbon nanostructureshaving tubular walls create undesirable strain that can affectstructural integrity and other properties of the nanostructure. Forexample, reports in the literature suggest that kinks in the ring shapednanotubes prevent formation of carbon nanostructures smaller than 70 nmin diameter. In any event, the terms “carbon nanoring” and “carbonnanostructures” shall exclude ring-like structures formed by joiningopposite ends of a carbon nanotube.

In addition to good electron transfer, the carbon nanostructures of thepresent invention have high porosity and large surface areas. Adsorptionand desorption isotherms indicate that the carbon nanostructures form amesoporous material. The BET specific surface area of the carbonnanostructures can be between about 80 and about 400 m²/g and ispreferably greater than about 120 m²/g, and typically about 200 m²/g,which is significantly higher than the typical 100 m²/g observed forcarbon nanotubes.

The high surface area and high porosity of the carbon nanostructuresmanufactured according to the present invention makes the carbonnanostructures useful as a support material for nanoparticle catalysts.Improved diffusion of reactants and/or electrons through the supportmaterial improves the efficiency with which substrates and electrons canbe transferred to the catalytic surface of the nanoparticles.Consequently, the supported catalysts of the present invention performbetter than nanoparticles supported on traditional supports such ascarbon black.

As discussed in U.S. application Ser. No. 11/351,620, filed Feb. 9,2006, the disclosure of which is incorporate herein, another use forcarbon nanostructures manufactured according to the invention is as asolid particulate filler material added to a polymeric material (e.g.,as a replacement for carbon black or carbon nanotubes). Preliminarytesting of polymeric materials that were filled with carbonnanostructures according to the invention indicates that such filledpolymeric materials have substantially reduced surface resistancecompared to polymers filled with a comparable quantity of carbon blackor carbon nanotubes.

V. Examples

The following examples provide formulas for making carbon nanostructuresaccording to the present invention.

Example 1

Example 1 describes a method for making carbon nanostructures usingsolid catalytic nanoparticles. A 0.1 M iron solution was prepared using2.24 g iron powder, 7.70 g citric acid, and 400 ml water. Theiron-containing mixture was mixed in a closed bottle on a shaker tablefor 7 days, with brief interruption (e.g., 1-2 minutes) to open thecontainer to vent hydrogen and allow air into the vapor space in thebottle. 100 ml of the iron solution was slowly added to a mixture of6.10 g of resorcinol and 9.0 g of formaldehyde. 30 ml of ammoniumhydroxide was added drop-wise with vigorous stirring. The pH of theresulting suspension was 10.26. The slurry was then cured at 80-90° C.(oil bath) for 3.5 hours to form an intermediate carbon nanostructure.The intermediate carbon nanostructure was collected by filtering andthen dried in an oven overnight and then carbonized at 1150° C. under N₂flow for 3 hour. The resulting composite nanostructure was refluxed in5M HNO₃ for 6-8 hours and then treated with 300 ml of mixture(H₂O/H₂SO₄/KMnO₄, molar ratio=1:0.01:0.003) at 90° C. for 3 hours.Finally, the carbon nanostructures were washed with water, and dried inan oven for 3 hours. The procedure yielded 1.1 g of carbon nanostructureproduct (i.e., carbon nanorings and/or hollow multi-walled sphere-likestructures).

The carbon nanostructures manufactured in Example 1 were then analyzed,first by TEM and later by SEM. TEM images of the nanorings from Example1 are shown in FIGS. 1A-1C, 2A-2B and 3A-3B. As seen in the TEM images,the method of the present invention can produce carbon nanostructuresthat appear to be predominantly ring-shaped (i.e., “nanorings”) andnanostructures of uniform size. The SEM images of FIGS. 4A-4B of thesame carbon nanostructures indicate that the nanostructures are actuallysphere-like (or spheroidal) rather than ring-shaped. Because thesphere-like multi-walled carbon nanostructures have a hole in themiddle, as shown by the TEM images, they are not “nano onions”, whichare solid.

Example 2

In example 2, carbon nanostructures were manufactured in a processsimilar to Example 1, except that the intermediate carbon nanostructureswere carbonized at 850° C. for 4 hours. The procedure yielded 1.04 g ofcarbon nanostructure product (i.e., sphere-like multi-walled carbonnanostructures and/or carbon nanorings).

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

1. A composition of matter comprising a plurality of carbonnanostructures having a graphitic structure and a BET specific surfacearea greater than about 120 m²/g, the carbon nanostructures beingmanufactured according to a process comprising: (i) forming a pluralityof catalytic templating nanoparticles by: (a) reacting a plurality ofprecursor catalyst atoms with a plurality of organic dispersing agentmolecules to form complexed catalyst atoms; and (b) allowing or causingthe complexed catalyst atoms to form the templating nanoparticles; (ii)forming intermediate carbon nanostructures by polymerizing a carbonprecursor in the presence of the templating nanoparticles; (iii)carbonizing the intermediate carbon nanostructures to form a pluralityof composite nanostructures; and (iv) removing the templatingnanoparticles from the composite nanostructures to yield the carbonnanostructures.
 2. A composition of matter as defined in claim 1,wherein the precursor catalyst atoms used to form the catalytictemplating nanoparticles comprise at least one of iron, nickel, orcobalt.
 3. A composition of matter as defined in claim 1, wherein thedispersing agent molecules used to form the catalytic templatingnanoparticles are capable of bonding with the precursor catalyst atomsand comprise at least one functional group selected from the groupconsisting of a hydroxyl, a carboxyl, a carbonyl, an amine, an amide, anitrile, a nitrogen with a free lone pair of electrons, an amino acid, athiol, a sulfonic acid, a sulfonyl halide, an acyl halide, andcombinations thereof.
 4. A composition of matter as defined in claim 1,wherein the dispersing agent molecules used to form the catalytictemplating nanoparticles comprise at least one member selected from thegroup consisting of oxalic acid, malic acid, malonic acid, maleic acid,succinic acid, glycolic acid, lactic acid, glucose, citric acid,pectins, cellulose, ethanolamine, mercaptoethanol, 2-mercaptoacetate,glycine, sulfobenzyl alcohol, sulfobenzoic acid, sulfobenzyl thiol,sulfobenzyl amine, polyacrylates, polyvinylbenzoates, polyvinyl sulfate,polyvinyl sulfonates, polybisphenol carbonates, polybenzimidizoles,polypyridine, sulfonated polyethylene terephthalate, and combinationsthereof.
 5. A composition of matter as defined in claim 1, wherein thecarbon precursor used to form the intermediate carbon nanostructurescomprises a hydrothermally polymerizable organic substrate thatcomprises at least one of citric acid, acrylic acid, benzoic acid,acrylic ester, butadiene, styrene, or cinnamic acid.
 6. A composition ofmatter as defined in claim 1, wherein the carbon precursor used to formthe intermediate carbon nanostructures comprises at least one ofresorcinol-formaldehyde-gel, phenol resin, melamine-formaldehyde gel,poly(furfuryl alcohol), or poly(acrylonitrile).
 7. A composition ofmatter as defined in claim 1, wherein the templating nanoparticles areformed prior to being mixed with the carbon precursor.
 8. A compositionof matter as defined in claim 1, wherein at least a portion of thetemplating nanoparticles are removed from the composite nanostructuresby etching with at least one of an acid or a base to yield the carbonnanostructures.
 9. A composition of matter as defined in claim 1, thecarbon nanostructures being composed of hollow multi-walled structures,each multi-walled structure being formed from multiple graphitic layers.10. A composition of matter as defined in claim 1, wherein (a) furthercomprises mixing and reacting a ground state metal comprising theprecursor catalyst atoms with the organic dispersing agent molecules toform the complexed catalyst atoms and adding a base to adjust the pH toabove 8 and below about
 13. 11. A composition of matter as defined inclaim 1, further comprising catalyst particles on the carbonnanostructures.
 12. A composition of matter comprising a plurality ofcarbon nanostructures having a graphitic structure and a BET specificsurface area greater than about 120 m²/g and being composed of hollowmulti-walled sphere-like carbon nanostructures, each multi-walledsphere-like carbon nanostructure being formed from multiple graphiticlayers and having a single interior hole defining an interior diameterof the sphere-like carbon nanostructure, the carbon nanostructures beingmanufactured according to a process comprising: (i) providing aplurality of solid catalytic templating nanoparticles consistingessentially of one or more types of metal catalyst atoms and optionallyone or more types of organic dispersing agent molecules; (ii) mixing thesolid catalytic templating nanoparticles with a carbon precursor andpolymerizing the carbon precursor in the presence of the solid catalytictemplating nanoparticles to form a plurality of intermediate carbonnanostructures; (iii) carbonizing the intermediate carbon nanostructuresto form a plurality of composite nanostructures; and (iv) removing thetemplating nanoparticles from the composite nanostructures to yield thecarbon nanostructures.
 13. A composition of matter as defined in claim12, wherein the metal catalyst atoms comprise at least one of iron,nickel, or cobalt and the dispersing agent molecules comprise at leastone functional group selected from the group consisting of a hydroxyl, acarboxyl, a carbonyl, an amine, an amide, a nitrile, a nitrogen with afree lone pair of electrons, an amino acid, a thiol, a sulfonic acid, asulfonyl halide, an acyl halide, and combinations thereof.
 14. Acomposition of matter as defined in claim 12, wherein the dispersingagent molecules comprise at least one member selected from the groupconsisting of oxalic acid, malic acid, malonic acid, maleic acid,succinic acid, glycolic acid, lactic acid, glucose, citric acid,pectins, cellulose, ethanolamine, mercaptoethanol, 2-mercaptoacetate,glycine, sulfobenzyl alcohol, sulfobenzoic acid, sulfobenzyl thiol,sulfobenzyl amine, polyacrylates, polyvinylbenzoates, polyvinyl sulfate,polyvinyl sulfonates, polybisphenol carbonates, polybenzimidizoles,polypyridine, sulfonated polyethylene terephthalate, and combinationsthereof.
 15. A composition of matter as defined in claim 12, wherein thecarbon precursor comprises a hydrothermally polymerizable organicsubstrate that comprises at least one of citric acid, acrylic acid,benzoic acid, acrylic ester, butadiene, styrene or cinnamic acid.
 16. Acomposition of matter as defined in claim 12, wherein the carbonprecursor comprises at least one of resorcinol-formaldehyde-gel, phenolresin, melamine-formaldehyde gel, poly(furfuryl alcohol), orpoly(acrylonitrile).
 17. A composition of matter as defined in claim 12,wherein the solid catalytic templating nanoparticles are provided in anaqueous medium having pH greater than 8 and less than about
 13. 18. Acomposition of matter comprising: a plurality of carbon nanostructures,the carbon nanostructures having a graphitic structure and a BETspecific surface area greater than about 120 m²/g, the carbonnanostructures being composed of hollow multi-walled sphere-like carbonnanostructures, each multi-walled sphere-like carbon nanostructure beingformed from multiple graphitic layers and having a single interior holedefining an interior diameter of the sphere-like carbon nanostructure.19. A composition of matter as defined in claim 18, at least some of thesphere-like carbon nanostructures further including a catalytictemplating nanoparticle disposed within the single interior hole.
 20. Acomposition of matter as defined in claim 18, further comprisingcatalyst particles on the carbon nanostructures.